Grade beams on pile caps
Grade beams on pile caps
(OP)
Do grade beams have to be deep enough to have bottom/beam below frost depth? Since they are not actually supported by the soil but rather by pile caps, so I was just wondering.
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RE: Grade beams on pile caps
You possibly could design for this upward force, but there would be very high uncertainty as to its magnitude.
RE: Grade beams on pile caps
RE: Grade beams on pile caps
Dik
RE: Grade beams on pile caps
After a few years, the soil may settle away from the underswide of the grade beams, making them less susceptible to frost heave damage. But you have no direct control over that and cannot rely on it.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Grade beams on pile caps
Kieran
RE: Grade beams on pile caps
I would be hesitant to use a void underneath as this could fill up with water and still create uplift forces when this freezes.
The additional depth can pay dividends in other ways such as removing the need for shear reinforcement in the grade beam and thus vastly reducing labour.
RE: Grade beams on pile caps
Dik
RE: Grade beams on pile caps
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I prefer a cushion which occupies the entire void space. It compresses when necessary, then bounces back to its original volume. There may be other products which satisfy this requirement, but GeoSpan is the one I have specified recently.
h
BA
RE: Grade beams on pile caps
RE: Grade beams on pile caps
We have expansive clays in our area, though not nearly as expansive as the soils found in the neighboring provinces of Saskatchewan and Manitoba where heaving of six or seven inches has been reported.
A cushion will not function adequately unless its thickness is at least twice the expected heave due to either frost or swelling soil or, alternatively, unless the grade beam is reinforced for the uplift pressure and is tied down at supports to resist the negative reactions.
I typically use 4" GeoSpan in the Edmonton area with no problems so far. For a project in Winnipeg, Manitoba, I used 6" GeoSpan and have heard of no problems to date. Grade beams were designed for a substantial uplift pressure in the event that the soil swelling exceeded the estimates. This could be considered double kill but it didn't really matter because the cost of the reinforcement was not significant.
In the 1950's, it was common to form grade beams with a 'V' shape on the bottom with the idea that the clay soil would cleave to each side when it heaved, hence prevent uplift. I never could understand that logic, but I cannot criticize it too much because some of those old buildings are still around today and are behaving reasonably well.
BA
RE: Grade beams on pile caps
I've done a lot of warehouses and pre-eng buildings using 24" or 30" grade beams using that approach and not had a problem.
JAE:
The Winnipeg area has hightly plastic clays for a depth of 30-40' typically. Fortunately, the soil is generally saturated and generally desicates shrinks a bit after construction.
Dik